1. Field of the Invention
The present invention relates to a method and an apparatus for determining whether or not synchronization has been successfully performed by detecting a synchronous pattern in demodulated data.
2. Description of Related Art
A reception device having a demodulation unit, etc. receives a receiving wave (modulated wave), and an pulling operation is performed according to a predetermined pulling sequence. An example of the pulling sequence is explained below by referring to FIGS. 1 through 3. In the present invention, the demodulation unit is a digital demodulator of the synchronous detection system.
FIG. 1 is a flowchart showing an example of an pulling sequence.
As shown in FIG. 1, in sequences (hereinafter referred to as “seq.”) 1 through 3, the adjustment control for allowing the oscillation frequency of a voltage control oscillator in a demodulation unit to track the carrier frequency of a receiving wave (modulated wave) is performed with low precision, intermediate precision, and high precision in this order. That is, the adjusting step is first performed roughly. Practically, a low-precision adjustment is first performed such that an adjustment can be made in a wide range, then an intermediate-precision adjustment is made, and finally a high-precision adjustment is made, thereby performing the adjustment (pulling).
Next, in seq. 4, a synchronous pattern (for example, a pattern formed by synchronous bytes) is detected from the demodulated data input from the demodulation unit, and it is determined whether or not frame synchronization has been successfully performed.
Detecting a synchronous pattern from demodulated data as described above is described in, for example, the patent document 1 (Japanese Published Patent Application No. S58-178653).
In the determination in seq. 4, when a determination result is YES, control is passed to seq. 5, the pulling sequence is completed, and control is passed to a monitor sequence. When the determination result in seq. 4 is NO, control is returned to seq. 1, and the above-mentioned processes are repeated until the determination result in seq. 4 becomes YES.
In the above-mentioned pulling sequence, when the synchronous pattern detected from the demodulated data by the frame synchronization determination above in the seq. 4 is assumed to be the synchronous bytes in the bit stream of frames, wherein one packet are formed by 204 bytes and each frame is formed by one packet having synchronous byte of 0×B8 as a leading byte and seven packets each having synchronous byte of 0×47 as a leading byte, in the determination in seq. 4, it is necessary to detect all synchronous bytes in the certain number of continuous frames to avoid erroneous determination. In this case, it is normally considered that it is necessary to detect all synchronous bytes in the five or more continuous frames.
In the frame synchronization determination in seq. 4, although it is determined that the frame synchronization is successfully performed if all synchronous bytes can be detected in five continuous frames, the period assigned to the determination in seq. 4 is set to approximately double the shortest possible time in which the frame synchronization could be successfully performed from the start of determination (in this example, the time required to detect all synchronous bytes in the five continuous frames) to include a margin because the frame synchronization is not necessarily performed successfully immediately after the start of the process in seq. 4. In detecting the five frames, the data of 204*8*5=8160 bytes is necessary. To collect the data of 8160 bytes, 10 ms are required in the system condition of 16 QAM (quadrature amplitude modulation) and 1.6 Mbaud.
In the sequence design in the above-mentioned pulling sequence, when it takes some time to determine whether or not the frame synchronization has been successfully performed, the period of the pulling sequence is restricted.
For example, in the pulling sequence shown in FIG. 1, if 40 ms, 20 ms, 20 ms, and 20 ms are respectively assigned to seq. 1, seq. 2, seq. 3, and seq. 4 as shown in FIG. 3, then the long time of 20 ms (20%) out of the total time of 100 ms (in one period) for seq. 1 through 4 is used in determination of seq. 4, and there is only 80 ms (80%) for adjustment of low, intermediate, and high precision for seq. 1 through 3. The synchronization signal shown in FIG. 3 indicates “H” when a condition with which it can be determined that frame synchronization has been successfully performed (for example, that all synchronous bytes in five continuous frames are detected) is satisfied. Otherwise, it indicates “L”. Therefore, it is determined by detecting the synchronization signal of “H” that frame synchronization has been successfully performed. In the example shown in FIG. 3, it is determined that frame synchronization has not been successfully performed in the frame synchronization determination in the first period, but that frame synchronization has been successfully performed in the frame synchronization determination in the second period.
As described above, in the frame synchronization determination in seq. 4, it is necessary to detect all synchronous bytes in the certain number of continuous frames, and the time longer than the shortest time required to determine that frame synchronization has been successfully performed after starting the determination is assigned to seq. 4. Therefore, when the time assigned to seq. 1 through 4 (one period) is predetermined, the time assigned to an adjustment with low, intermediate, and high precision for seq. 1 through 3 becomes relatively shorter when the time required to detect a frame in seq. 4 becomes longer, and there can be the possibility that a complete adjustment is not made. Therefore, when a complete adjustment is not made, the probability that frame synchronization is successfully performed becomes low, and the time (pulling operation time) required to complete an pulling sequence becomes long.